While the mouse is an excellent solution to the problem of controlling the motion of a cursor on the computer screen, it is poorly suited for simulating a writing implement such as a pen that can be used to input handwriting and the like. A number of pen-like pointing devices have been introduced to fill this need. One class of pen devices uses a stylus or similarly shaped object that is held like a pen such that its end moves over a special surface. The position of the pen on the surface is detected by sensors associated with the surface, and the position data is sent to the data processing system attached to the device. While such devices more nearly emulate a pen, they require a special writing surface that includes the sensors needed to track the stylus's motion. The cost of such systems is much greater than the cost of an optical mouse, and hence, these systems have not found wide spread acceptance. In addition, the tablets on which the user “writes” are of a fixed size. If the tablet is large, it is not suitable for portable computers. Similarly, if the tablet is small, the amount of data that can be written is significantly reduced.
Systems based on tracking the location of the pen on a surface such as a white board are also known to the art. These systems determine the position of the pen with the aid of two receivers that are placed on or near the writing surface. The pen includes an ultrasound transmitter and an infrared transmitter. Each of the receivers includes a matching ultrasound receiver and infrared receiver. When the pen makes contact with the writing surface, an ultrasound pulse and an infrared pulse are emitted by the pen. Each of these signals is detected by the receivers. By comparing the difference in arrival times of the infrared pulse and ultrasound pulse at each of the receivers, the location of the pen can be ascertained.
This basic tracking system does not, however, provide any information with respect to the rotation position of the pen. In some applications, observing the rotation of the pen can be useful. For example, the user could rotate the pen in a predetermined direction to signal an event such as a mouse click. A rotation in one direction could implement a right button click and a rotation in the other direction could implement a left button click.